This invention relates to a process for producing powdery polycarbonate from a polycarbonate solution.
Numerous attempts have been made to obtain powdery polycarbonate from a polycarbonate solution. For example, there is a process which comprises charging a polycarbonate solution into a cylindrical casing in having one or two cooperating screws engaging each other, evaporating the solvent present in the solution while the solution is moved through the casing by the screws to obtain melt polycarbonate. However, in this process, when the solvent is blown out from the solvent exit of the casing, some of the polycarbonate solution is entrained with the solvent vapor, and this causes clogging of the solvent exit. Even when many exits are installed in the longitudinal direction of the casing, most of the exits become partially or completely clogged. This increases the internal pressure of the casing whereby the polycarbonate solution is caused to back-flow into the feed opening for the solution.
In order to overcome this shortcoming, a process for obtaining polycarbonate in a powdery state has been proposed. For example, Japanese Patent Publication (laid open) No. 41048/1976 discloses a process in which a desolvating apparatus is used, having two screws with particular helical blades, the screws being arranged to rotate inward. The evaporating zone is the space between the inner wall of the cylindrical container and the upper portion of the screws as shown in the drawings of the above Publication. Since the apparatus is provided with an evaporating zone, clogging of the solvent exits is eliminated. However, in this process, the solid polycarbonate is deposited on the inner side wall from which the periphery of screws begin to depart. In addition, since a progressive blade and retrogressive blade are used, the efficiency of the desolvating apparatus is lowered. As a result, much solid polycarbonate accumulates on the wall of the apparatus. Therefore, the concentration gradient of the polycarbonate solution from the feed opening for the solution to the exit for the product becomes non-uniform, and eventually polycarbonate solution which is not sufficiently concentrated is discharged from the product exit .